California Route 110
Length: 8.2 miles (Pasadena Fwy.) and 2.3 miles (Harbor Fwy.)
Termini: I-10 exit 13 in downtown Los Angeles, and the Arroyo Parkway just after entering Pasadena
Reason for being a state route: The Pasadena Freeway segment predates the first construction of the Interstate system by 16 years (it was built in 1940). It's technically BELOW Pennsylvania standard, but that leads to its charm. As for the Harbor Freeway segment between US 101 and I-10, I do not find any good reason to make that a state route, other than "well, I-110 ends at I-10". But extending I-110 to the Fourl Level is a non-violating idea.
History: The Pasadena Freeway was originally the Arroyo Seco Parkway. It was built in 1940, hence the problems with geography that technology then could not master. Most of the ramps are 5 MPH ones, and the speed limit (design speed, probably) is 55 MPH. Before being CA 110 in 1984, this route was previously CA 11 (1940-1984) and US 66 (194?-1964).
The Harbor Freeway, on the other hand, was built in the 1950s. Originally US 6 followed the Harbor and then some city streets, eventually ending at the Los Alamitos Traffic Circle, where the Pacific Coast Highway (now CA 1) intersected what is now CA 19. But that section became I-110 in 1984. The only reason for CA 110 is continuity, basically.